In commercial cooking equipment having electric heating elements, electrically actuated contactors are connected in series with the heating elements to make or break power circuits through them. A first temperature control device opens and closes one of the contactors to deenergize and energize the heating elements in order to maintain the cooking temperature desired by the operator. In addition, a second temperature control device serves as a safety control to open the other contactor and deenergize the heating elements should some malfunctioning of the first control device occur and the cooking temperature exceeds the determined safe limit.
U.S. Pat. No. 3,824,373 issued July 16, 1974 to Clarence H. Napier illustrates an operating control circuit for a deep fat fryer having these components, but the safety thermostat and contactor are not energized on a regular basis so that malfunctioning thereof could occur without detection to frustrate the safety control and even allow damage and/or a fire as a result.
U.S. Pat. No. 3,919,523 issued Nov. 11, 1975 to Wadia et al. teaches an improved circuit where the safety contactor is operated each and every time main power connected to the unit is interrupted by a manual stop control. This tests the safety control regularly, in effect, every day when the unit is shut off. An indicator light connected across power lines between the safety contactor and the heating elements is illuminated in the safety contactor closed condition to apprise the operator of the faulty safety control.
Two embodiments are disclosed in the Wadia et al. patent. The first embodiment includes a one way safety thermostat device that requires manual resetting of the contacts once they have been shifted responsive to an overheat condition, and the second embodiment provides a pair of relays that respectively maintain the operating circuit energized until an overheat condition occurs and maintain the safety circuit energized thereafter until a manual reset step occurs. This manual reset feature is a major advantage of the safety control since it renders the appliance inoperative once an overheat condition occurs and the operator is thereby apprised of the overheat situation.
Using a reset type thermostat by itself on certain cooking appliances does present an additional safety problem because of the location required for the reset button. For example, a standard hydraulic thermostat device has a switch contact assembly including an actuating bellows and tubing connecting the bellows to a temperature sensing bulb. In a deep fat fryer where the heating elements are intended to be covered by cooking fat, the sensing bulb is carried on the heating element to detect both the fat temperature and the temperature of the heating elements in a no or low fat condition. To allow for easy cleaning of the cooking container, the heating elements are mounted from a movable rear housing of the fryer and this requires the contact assembly also to be mounted on this movable housing to avoid repeated flexing of the connecting tubing between the bulb and bellows. With the thermostat reset button mounted on the rear fryer housing, the operator must reach across and over the hot cooking fats in order to manipulate the reset button.
The relay control allows varied and safer mounting locations of the manual on/off reset actuators relative to the physical location of the thermostats since only flexible electrical wires are required to operatively connect the components together. The previously patented Wadia et al. control relay concept, however, requires two such relays.